Conventional articles of athletic footwear generally include two primary elements, an upper and a sole structure. The upper is secured to the sole structure and forms a void on the interior of the footwear for comfortably and securely receiving a foot. The sole structure is secured to a lower portion of the upper and is positioned between the foot and the ground. The sole structure generally incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole, or sockliner, is a thin, compressible member located within the void and proximate a lower surface of the foot to enhance footwear comfort. The midsole, which is conventionally secured to the upper along the length of the upper, forms a middle layer of the sole structure and is primarily responsible for attenuating ground (or other contact surface) reaction forces to lessen stresses upon the foot and leg. The outsole forms a ground-engaging portion (or other contact surface-engaging portion) of the sole structure, and is formed from a durable and wear-resistant material that includes texturing to improve traction.
The conventional midsole is primarily formed from a resilient, polymer foam material that extends throughout the length of the footwear, often by way of a molding process. The properties of the polymer foam material in the midsole are primarily dependent upon factors that include the dimensional configuration of the midsole and the specific characteristics of the material selected for the polymer foam, including the hardness or density of the polymer foam material. By varying these factors throughout the midsole, the relative stiffness and degree of ground reaction force attenuation may be altered to meet the specific demands of the activity for which the footwear is intended to be used. In addition to polymer foam materials, conventional midsoles may include, for example, one or more fluid-filled bladders and moderators.
In many articles of footwear, the midsoles are formed of two or more materials of different colors. In the molding of such midsoles, the two different materials may bleed together during the manufacturing process, which reduces the aesthetic appeal of the footwear.
It would be desirable to provide a midsole formed from two preforms and a method and mold for manufacturing such a midsole that reduces or overcomes some or all of the difficulties inherent in prior known devices. Particular advantages will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain embodiments.